Another train journey, another chance to run good ole Netstumbler and do a survey of channel occupancy for 2.4 GHz (that's 802.11b, g and n and not 802.11a in case you were wondering) to see whether my previous analysis of which are the best WiFi channels to use still holds.

For those who haven't (or can't be bothered to) read my previous article, I came to the conclusion that if you lived in an area of high WiFi penetration, channel 1 was the best channel to use as it was the least likely to suffer interference from other Wireless LAN users. In areas where there was unlikely to be any other wireless LAN activity, channel 11 (or 12, or 13) would be best, as these are the most free from other interferers (e.g. the military, microwave ovens, radio amateurs and so forth).

So what are the results of this train journey? I've plotted them above. I've shown the outbound journey separate from my return journey. As it's highly possible that if I picked up a LAN in one direction, I might have equally picked it up in the other, I've filtered the return numbers to take account of this. Also, I kind of half forgot to switch my system on on the outbound journey so, as you can see, the results for the return journey show many more LAN's than the outbound!

The upshot remains exactly the same as before (phew!) Channel 1 continues to be the best channel to use if you are in an area saturated with other users. Remember when looking at the above graph that channels 2 to 5 interfere with channel 1 and as such are not independent - equally they interfere wich channel 6 - only channels 1, 6 and 11 (or 1, 7 and 13) are actually free from interference from each other. My arguments about channel 11, 12 or 13 being the best to use in quiet areas remain unchallenged.

As a postscript, I though you might enjoy one or two of the network SSID's (names) that I found during my journey. Here are my favourites:

and my absolute favourite: 'FRAUDULENT'...! Also, a few other vaguely interesting facts and figures:

Number of networks called 'BTVOYAGER': 12Number of networks called 'BTHomeHub': 45Number of networks called 'SKYxxxxx' (where xxxxx is a 5 digit number): 30Number of networks called 'Belkin54g': 10 (and 7 of them were open)Number of networks called 'default': 5 (all of them open)Number of networks called 'linksys': 10 (4 of them were open)Number of networks called 'NETGEAR': 16 (10 of them were open)

In a previous post I discussed the fact that the powers that be in Europe had taken a decision with respect to low power, licence exempt radio equipment that meant that the 'heart' of the 70 centimetres amateur radio band had been ripped out due to a mountain of noisy interference produced by wireless doorbells, weather stations and so on.

But that's not the end of the story, or so it would seem. Not content with annoying noisy oysters by wiping out 433.500 MHz (the EU 70cms FM calling channel) as well as the repeater input and output frequencies in the UK (as well as Finland, Slovenia and certain other countries) from 433.000 to 433.375 paired with 434.600 to 434.975 MHz, I recently discovered that they have also managed to pull the wool over our eyes with respect to another travesty against 70 cms radio amateurs. But this time, it's not those of us in Europe that are suffering, it is amateurs in the USA and in particular, in areas where European tourists congregate.

What would be the worst possible frequency to interfere with? Probably the calling frequencies which in Europe are 433.500 for FM (already messed up with the aforementioned low power devices) and 432.200 for SSB. But hang on... US amateurs have a wider 70cm allocation, from 420 to 450 MHz and... their FM calling frequency is 446.000 MHz. So what type of wool have the ERO pulled over the eyes of US amateurs then? PMR 446 of course! Yes, this low power (half a Watt) licence exempt walkie-talkie technology runs on frequencies from 446.000 to 446.100 (and recently extended to 446.200 for digital modulation) - slap bang in the middle of the US 70 cms FM calling and working channels.

Now obviously, there's little, or no chance, of UHF signals propagating from Europe to the US, so the likelihood of European PMR 446 equipment causing a problem to American amateurs is nil isn't it? Well, if the equipment is used in Europe there is no chance of interference. However, if it's taken to the US by European holidaymakers to allow them to keep in touch with each other whilst on the beach, at a theme park, or even whilst lost amongst the endless miles of aisles at Wal*Mart, then yes! And this is just what is happening. Europeans, blissfully unaware that their equipment is operating illegally once taken outside the EU, are using PMR 446 equipment in the USA (and Canada) and, in the process, producing endless interference on the main FM channels.

Somewhere in the basement of the ERO, CEPT or similar, someone is having jolly good fun thinking up increasingly intricate wheezes for messing up the 70 cms amateur bands. What next? How's about sharing the band with a very high powered radar that wipes out reception across most of Northern Europe? Oh, I forgot, they've already done this haven't they...!

These days, a lot of radio planning (broadcasting and cellular for example) relies heavily on the ability to use computers to predict coverage from a site. So Wireless Waffle thought it would prove an interesting exercise to do some coverage predictions from a known radio station and compare them with actual reception. It's probably worth adding that using a single receive antenna in a fixed location for comparison isn't necessarily fair - reflections could mean that there are strong nulls in a particular direction, though with the antenna mounted nice and high and above most nearby obstacles these effects are minimised.

The prediction tool used is RadioMobile, which is a freeware tool designed for radio amateurs and does pretty good VHF and UHF predictions as well as path profiles which can be used to hazard a guess at microwave links too. The 70 centimetre amateur band was used as the test case and the signals from various neighbouring repeaters as the benchmark. Most repeaters have decently sited antennas at a reasonable height and free of local obstructions and so should make for a reasonably accurate prediction.

First off, a scan around the 70cm (433 MHz) band on a day when propagation is flat was conducted, to see which repeaters could be heard and what strength they were. Here's the list (channels not listed are ones on which nothing was heard):

So the question is, could a computer prediction reproduce this set of results? The parameters of the station plus those of all the nearby repeaters (including those that are nearby but which weren't received) were entered into RadioMobile and a plot produced. The results are shown on the right (click to see the full size version).

The easiest way to see what the results show is to look at the colour of the line between the amateur station used and the various repeaters. A solid red line indicates that the prediction shows there to be no possibility of a signal getting through; yellow indicates that reception would be on the fringes of possibility and green indicates that all should be OK. A dotted line shows that the prediction is borderline between two possible outcomes. So here's what the prediction says (ignoring all those which are purely red and thus should be inaudible):

A number of predictions have been highlighted in bold as these are at variance with what was borne out in practice. It seems there are 4 other repeaters which should be audible that aren't. Also, GB3BV is shown as very marginal on the prediction yet is a reasonably good signal. However, the prediction has correctly identified approximately the correct likelihood of reception for 9 of the original 10 repeaters being received.

So are predictions any good? From the results shown here, they're not bad, however the relatively simple predictions produced by RadioMobile are probably insufficiently complex to take account of other factors that play a major role in radio propagation such as clutter from trees and buildings. Nonetheless, not a bad result and an interesting illustration of the power of computers.

So you've bought your iTrip, micro FM transmitter, AirPlay, PodFreq or similar and plugged it into your iPod or PSP and managed to get it to transmit somewhere in the FM band. You even manage to find a clear FM frequency at home where you don't suffer interference from local radio transmitters (or your neighbours' iTrips!) Then you decide to go on a road trip and you take it into your car. Driving around the UK you soon find that the FM frequency that was clear at home is home to a neighbouring radio station and that as you drive along, reception of your iTrip is blighted by interference from local radio stations (how dare they!)

What you need is a frequency somewhere in the FM band that is clear of licensed stations so that you can drive up and down the country without ever suffering interference or having to re-tune your iTrip. Dream on! There are only nine frequencies in the mainland UK which are not used by high powered local (or national) radio stations, these being 87.5, 87.6, 87.7, 87.8, 87.9, 88.0, 105.0, 105.5 and 108.0 MHz. If you include pirate radio stations on the list, there are virtually no clear frequencies at all - Shine on 87.9, Point Blank on 108.0 and UK's Finest on 87.5 being good examples of stations that occupy these seemingly clear channels. However to get clear, interference-free reception it's wise to have at least 200 kHz between you and any other station. Pirates aside, this means that 88.0, 105.0, 105.5 and 108.0 are out leaving only 87.5 - 87.9 MHz. This 'clear' spectrum is not, however, unused: 87.7 and 87.8 MHz are the most common frequencies for low-power FM stations, either short term (RSL) stations, or the new wave of community radio stations.

So what to do? Well excluding one or two pirates, using 87.5 MHz is a fairly safe bet, unless you happen to live in a major city where pirates are prevalent or near a long-term RSL or community station on 87.7. But what about elsewhere on the FM dial? Are there any 'cold-spots' where there is a smaller likelihood of coming across an interfering station.

The main BBC sub-bands (88.1 - 90.2 for Radio 2, 90.3 - 92.4 for Radio 3, 92.5 - 94.6 for Radio 4, and Radio Scotland and 97.7 - 99.8 MHz for Radio 1) are pretty chocker-block and many of the transmitters are very high power (250 kW is not totally uncommon) so they are not a good place to look. The other BBC sub-bands, 94.7 - 96.0 and 103.5 - 104.9 MHz or thereabouts, used for BBC local radio, or BBC Radio 4 in Scotland, BBC Wales and BBC Cymru and are also shared with independent local radio (ILR) in places, are pretty busy too. Not doing too well so far... However, an analysis of the ILR bands (96.1 - 97.6 and 99.9 - 103.4 MHz and 105.0 to 107.9) shows some interesting anomolies.

In the lower of these two bands, the frequency 96.8 is only used twice (though it is home to a whacking 250kW BBC Cymru transmission in Wales) and 96.1 and 97.3 are only used 3 times.

In the range 99.9 to 103.4, 99.9, 100.6 and 102.1 MHz are only used once (though the band 99.9 to 102.0 is repleat with very high powered Classic FM transmitters) and there are several frequencies only used twice across the UK. Finally, in the range 105.0 to 107.9, the frequencies 105.0 and 105.5 MHz are not used at all, and the frequencies 105.1, 105.3, 105.9, and 106.5 MHz are only used once.

Although this analysis is based on slightly old data (2005) published by Ofcom it does tend to suggest that in addition to 87.5, there are other frequencies which might provide relatively interference free iTrip usage across the UK without needing to re-tune. Unless your local station happens to be one one of these frequencies (or one adjacent to it), I would suggest 99.9, 105.0 (or 105.1) and 106.5 MHz as possible alternatives.